Gems as Treasure and Merchandise

If the PCs find a gem as treasure in the dungeon, and try to sell it back in town, does the demand modifier for gems and/or semi-precious stones in the town affect the price they get for the gem? Or does that only apply to gems and semi-precious stones bundled into loads? If so, can the PCs collect the gems they find as treasure into boxes worth 1000 or 3000 gp and treat them as loads of merchandise?
I realize this is a DM’s call kind of situation. I was just wondering how you guys ran your game.

Sounds reasonable to me, if you don’t mind the paperwork. I always liked rewarding my Traveller players for thinking things through and going out of their way to buy ship parts on, say, an industrial planet rather than a backwater, despite the fact that there were no explicit rules for them being cheaper there.

For the sake of convenience, I usually assume that the gp value listed for gems is relative to whatever the closest market is. So, for example, if the trade goods table shows there is a high demand for gems, the “1000gp gem” they find in a dungeon is probably a smaller gem than the “1000gp gem” found in dungeons where there is low demand for gems nearby.
If the players make the effort to cart their gems to a high-demand region I will let them use the trade rules, of course.